This invention relates to dynamo-electric machines such as electric motors generator and the like and, more particularly, to a method of controlling flux in a brushless permanent magnet (BPM) motor or generator.
In permanent magnet motors and generators, there is a constant level of flux which is fixed by magnets comprising part of the motor or generator assembly. This is significant because, for example, if the motor or generator has a certain level of torque at a certain speed or range of operating speeds, the motor or generator must be specifically designed to properly function with this set of operating requirements. Thus, if the motor is to have constant torque at high speeds, the motor may have fewer turns in its armature. If there are fewer turns, there is correspondingly a higher current level which must be accommodated by an inverter doing the switching between motor phases. That is, the inverter will need switching elements having greater current carrying capacity. This increases the cost of the inverter. It would be advantageous in brushless permanent magnet motors (or generators) to provide a flux control capability by which phase switching in the motor or generator can be accomplished by a converter of conventional design. This also produces a more efficient dynamo-electric machine.
One approach to doing this could be based upon the dynamo-electric machine disclosed by Bekey and Robinson in their U.S. Pat. No. 2,796,542. As taught by Bekey and Robinson, machine flux is controllable by defining a specific, isolated rotor pole structure. This results in a magnetic circuit which produces a particular set of motor operating characteristics. However, to use an approach based on this machine design would require improvements in machine efficiency.
An enhanced flux control capability for use in motors has an application in appliances; for example, in washing machines. These are typically high speed, low torque applications of a BPM motor. To achieve high speed, low torque, so many turns have to be taken out of the motor that sufficient current cannot be achieved at low speed, high torque motor conditions to provide efficient operation. This problem can be recitified by changing the design of the inverter used with the motor but there are costs and other penalties associated with doing so.
As a generator, a flux controlled dynamo-electric machine has certain advantages over the Lundell type alternator or generator presently used in automobiles. With the ever increasing enhancements in automobiles, both with respect to performance and passenger comfort, there is a need for an alternator capable of supplying more electricity than the current type alternators are able to provide. A particular problem in this area, of course, is that the generator output must be constant over a wide range of generator operating speeds. A flux controllable machine, used with appropriate microprocessor or similar controls could fulfill the current requirements regardless of how fast the generator is running.